Define both element dimensions based on one parent dimension

Question

:root {
    --success: #0c0;
}

button {
    border: 2px solid;
    border-radius: 10px;
    cursor: pointer;
    margin: 1em 0.5em;
    padding: 10px 15px;
    transition: transform 1s;
}

button:active {
    transform: scale(90%);
}

button.animated {
    background-color: transparent;
    overflow: hidden;
    position: relative;
    transition: color 1s, border-color 1s, transform 0.2s;
    z-index: 1;
}

button.animated:hover {
    border-color: transparent;
}

button.animated::after {
    border: 0;
    border-radius: 50%;
    content: "";
    height: 150%;
    left: -25%;
    opacity: 0;
    position: absolute;
    top: -25%;
    transform: scale(0.1);
    transform-origin: center;
    transition: all 1s;
    width: 150%;
    z-index: -1;
}

button.animated:hover::after {
    opacity: 1;
    transform: scale(1);
}

.text,
button.animated.background::after {
    background-color: white;
}

.background,
button.animated.text:hover{
    color: white;
}

button.animated.text {
    border-color: var(--muted);
}

.success.text,
button.animated.success.background:hover {
    color: var(--success);
}

.success.background,
button.animated.success.text::after {
    background-color: var(--success);
}

button.animated.success.text:hover,
button.animated.success.background {
    border-color: var(--success);
}

<button class="animated success background">Button</button>
<button class="animated success background">Longer button</button>

The button in the snippet above has a background effect in the shape of an ellipse that appears when you hover over it. Currently, the dimensions of the ellipse are proportional to the dimensions of the button. I would like the ellipse to be circular, with its width 141.4% times the longer dimension of the button, or, if not possible, the width of the button. How can I achieve this?

Temani Afif · Accepted Answer · 2025-04-28 21:43:19Z

1

Here is an idea using background and gradients

button {
  border: 2px solid red;
  border-radius: 10px;
  cursor: pointer;
  margin: 1em 0.5em;
  padding: 10px 15px;
  background:
   radial-gradient(circle closest-side,#fff 98%,#0000) 
    50% padding-box no-repeat red;
  background-size: var(--s,0%) 999px; /* the second value is a very big value */
  transition: .5s;
}
button:hover {
  --s: 110%; /* you can use 100% but a bit bigger is better */
}

<button>Button</button>
<button>Longer button</button>

answered Apr 28 at 21:43

Temani Afif

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2 Comments

zer00ne Apr 28 at 23:10

This is cool, it's going in my snippets, what is this effect called?

Eric Wang Apr 29 at 16:35

@zer00ne It's a background image of a single circular radial gradient with only one color (so basically just a white circle) that is centered in the padding box (because of the 50%) and is as big as the shorter side of the background. Since the height of the background specified in background-size is "a very big value", this will almost always be the width, which is 0% long until the button is hovered over, at which point it becomes --s: 110% long. The background below the circle is red, so when you hover over the button, a white circle expands to cover the background.

Eric Wang · Accepted Answer · 2025-04-29 18:24:42Z

:root {
    --success: #0c0;
}

button {
    border: 2px solid;
    border-radius: 10px;
    cursor: pointer;
    margin: 1em 0.5em;
    padding: 10px 15px;
    transition: transform 1s;
}

button:active {
    transform: scale(90%);
}

button.animated {
    background-color: transparent;
    overflow: hidden;
    position: relative;
    transition: color 1s, border-color 1s, transform 0.2s;
    z-index: 1;
}

button.animated:hover {
    border-color: transparent;
}

button.animated::after {
    aspect-ratio: 1 / 1;
    border: 0;
    border-radius: 50%;
    content: "";
    left: 50%;
    opacity: 0;
    position: absolute;
    top: 50%;
    transform: translate(-50%, -50%) scale(0.1);
    transform-origin: center;
    transition: all 1s;
    width: 142%;
    z-index: -1;
}

button.animated:hover::after {
    opacity: 1;
    transform: translate(-50%, -50%) scale(1);
}

.text,
button.animated.background::after {
    background-color: white;
}

.background,
button.animated.text:hover{
    color: white;
}

button.animated.text {
    border-color: var(--muted);
}

.success.text,
button.animated.success.background:hover {
    color: var(--success);
}

.success.background,
button.animated.success.text::after {
    background-color: var(--success);
}

button.animated.success.text:hover,
button.animated.success.background {
    border-color: var(--success);
}

<button class="animated success background">Button</button>
<button class="animated success background">Longer button</button>

Solved it! I added aspect-ratio: 1 / 1;, set only the width to 142%, and simply used left: 50%;, top: 50%;, and transform: translate(-50%, -50%) to translate the background onto the center of the button. Added transform: translate(-50%, -50%) to :hover::after too, so the circle stays centered when the button is hovered over.

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